The excited states of many semiconducting nanostructures, such as carbon nanotubes (CNTs), are characterized by excitons, electron-hole pairs bound by Coulomb interactions (Wang, F. et al. (2005) “The Optical Resonances in Carbon Nanotubes Arise from Excitons” Science 308:838-841). Excitons are hydrogen-atom-like quasi-particles, each carrying a quantum of electronic excitation energy. An exciton can return to the ground state by emitting a photon, producing photoluminescence (PL), or by falling into a “dark” state from which the energy is lost as heat. The ability to control the fate of excitons and their energy is crucial to imaging (Hong, G. et al. (2012) “Multifunctional in vivo vascular imaging using near-infrared II fluorescence,” Nat. Med. 18:1841-1846; Chan, W. C. W. & Nie, S. (1998) “Quantum dot bioconjugates for ultrasensitive nonisotopic detection,” Science 281:2016-2018), sensing (Heller, D. A. et al. (2006) “Optical Detection of DNA Conformational Polymorphism on Single-Walled Carbon Nanotubes,” Science 311:508-511), photovoltaics (Kamat, P. V. (2008) “Quantum dot solar cells. Semiconductor nanocrystals as light harvesters,” J. Phys. Chem. C 112:18737-18753), lighting and display (Shirasaki, Y. et al. (2013) “Emergence of colloidal quantum-dot light-emitting technologies,” Nat. Photon. 7:13-23), and many other electronic applications.
Over the last few decades, two primary approaches have been developed to tailor the exciton properties within a nanocrystal—quantum confinement and doping. Quantum confinement has motivated the development of many synthetic approaches that control the size and shape of nanocrystals, and consequently their electronic and optical properties (Rossetti, R. et al. (1983) “Quantum size effects in the redox potentials, resonance Raman spectra, and electronic spectra of cadmium sulfide crystallites in aqueous solution,” J. Chem. Phys. 79:1086-1088; Alivisatos, A. P. (1996) “Semiconductor clusters, nanocrystals, and quantum dots,” Science 271:933-937; Yin, Y. & Alivisatos, A. P. (2005) “Colloidal nanocrystal synthesis and the organic-inorganic interface,” Nature 437:664-670). Doping modifies the electronic structure of the host crystal and the examples include nitrogen-vacancy in diamond (Gruber, A. et al. (1997) “Scanning confocal optical microscopy and magnetic resonance on single defect centers,” Science 276:2012-2014) and metal ion-doped nanocrystals (Erwin, S. C. et al. (2005) “Doping semiconductor nanocrystals,” Nature 436:91-94).
In the case of single-walled carbon nanotubes (SWCNTs), the excitonic properties depend on both the diameter and chiral angle of each nanotube crystal, collectively known as chirality, which may be denoted by a pair of integers (n,m) (see O'Connell, M. J. (2002) “Band Gap Fluorescence from Individual Single-Walled Carbon Nanotubes,” Science 297:593-596; Bachilo, S. M. (2002) “Structure Assigned Optical Spectra of Single-Walled Carbon Nanotubes,” Science 298:2361-2366). It has recently been demonstrated that the optical properties of SWCNTs can be modified by doping with oxygen (Ghosh, S. et al. (2010) “Oxygen doping modifies near-infrared band gaps in fluorescent single-walled carbon nanotubes,” Science 330:1656-1659) or by the incorporation of defects through diazonium chemistry (Piao, Y. et al. (2013) “Brightening of carbon nanotube photoluminescence through the incorporation of sp3 defects,” Nat. Chem. 5:840-845). These defects can induce a new near-infrared emission (Ghosh, S. et al. (2010) “Oxygen doping modifies near-infrared band gaps in fluorescent single-walled carbon nanotubes,” Science 330:1656-1659), brighten dark excitons (Piao, Y. et al. (2013) “Brightening of carbon nanotube photoluminescence through the incorporation of sp3 defects,” Nat. Chem. 5:840-845), facilitate up conversion (anti-stoke shift) (Akizuki, N. et al. (2015) “Efficient near-infrared up-conversion photoluminescence in carbon nanotubes,” Nat. Commun. 6), and stabilize trions at room temperature (Brozena, A. H. et al. (2014) “Controlled defects in semiconducting carbon nanotubes promote efficient generation and luminescence of trions,” ACS Nano 8:4239-4247), thus making them particularly interesting for emergent photonic applications. However, these conventional methods for defect creation have thus far been bound by the extremely limited chemical and optical tunability. In particular, oxygen doping leads to mixed ether and epoxide structures, and diazonium chemistry works only for specific aryl groups and monovalent bonding, and has relatively low reaction rates. Moreover, it has been demonstrated that diazonium chemistry and oxidative reactions occur on a SWCNT sidewall at completely random atomic sites (see Goldsmith et al. (2007) “Conductance-controlled point functionalization of single-walled carbon nanotubes,” Science 315:77-81); Cognet et al. (2007) “Stepwise quenching of exciton fluorescence in carbon nanotubes by single-molecule reactions,” Science 316:1465-1468). The covalent modification of even a single site utilizing such methodologies results in a substantial drop of electrical conductance (Goldsmith et al. (2007), Conductance-controlled point functionalization of single-walled carbon nanotubes,” Science 315:77-81) and stepwise quenching of exciton fluorescence in semiconducting nanotubes (Cognet et al. (2007), Stepwise quenching of exciton fluorescence in carbon nanotubes by single-molecule reactions,” Science 316:1465-1468). As such, prior methodologies utilizing defects pale in comparison with the large number of quantum dots that have been synthesized based on the quantum confinement effect. The use of defects for materials engineering has therefore not been achieved by such prior methodologies.
Accordingly, there is a need for new near-infrared emitters and synthetic approaches for creating such emitters that overcome some or all of the difficulties and limitations of conventional approaches.